Broadband amplifiers and power amplifiers, in particular, cause distortion to a signal, since the amplifiers function non-linearly at least to some degree. Besides distortion amplifiers cause for example noise. The influence of distortion caused by amplifiers has been reduced by means of feedforward amplifiers.
Usually, the incoming end and outgoing end of feedforward amplifiers have to be adapted to signal paths. Processing of signals always causes some delay to a signal. For this reason adaptation of the delay and gain requires particular accuracy so that the amplifier would function as desired in all operational environments. Prior art amplifiers have been part of an amplifier arrangement which has been provided with signal cancellation loops and error cancellation loops. The amplifier arrangement has adjusted the phase and amplitude of a signal acting on the loops, which has enabled reduction of distortion in the signal. The amplifier arrangement has also been able to adapt to the received signal to some degree.
Special factors, which can be denoted with .alpha. and .beta., are employed for adjusting the phase and amplitude. The factors are changed and adjusted by means of an adaptation factor, which is denoted with K. When the power level of an input signal changes rapidly, .alpha. and .beta. also need to change rapidly to correspond to the changed situation. In prior art solutions the same method has been used for adaptating both factors, which has caused problems. Problems have appeared particularly when a pilot signal has been used in adaptation.
A pilot signal, power minimization method and gradient method, for example, have been employed in adapting factors .alpha. and .beta.. However, the same method of adapting factors has been used in both cancellation loops, and thus any errors in the factors have been dependent on one another. The prior art methods have also suffered from other problems because the pilot signal has typically been used before signal amplification, in which case the pilot signal has caused noise and therefore it has been necessary to filter the signal out of the output of the amplifier.